Fluorescence Displays

Vacuum fluorescent displays (VFDs) are strongly related to flat-panel CRTs. Electrons are ejected from a cathode source, traverse a vacuum and then strike a pattern of triodes with individual anodes covered in red, green and blue phosphorescent material. However, the operating voltages, e.g. 12 V, and power consumption are much lower than those found for CRTs and PDFs. 

The fabrication costs of VFDs are also relatively low. They are rugged with long operating lifetimes. Therefore, small VFDs have been manufactured in large volume for several decades for a variety of applications, e.g. as part of car dashboards or orientation and navigation systems. 

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While some video display screens such as liquid crystal, gas plasma or vacuum fluorescent displays do not present the same charged screen hazards as CRTs, this does not imply that they are safe for use in hazardous locations. This requires special design and certification for use with a given flammable atmosphere. Non-certified equipment used in locations classified as hazardous under Article 500 of NFPA 70 National Electrical Code require a purged or pressurized enclosure to control ignition hazards as described in NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment. The screen in this case is located behind a window in the enclosure. 

Vacuum fluorescent displays (VFD) have been used since 1970. At the start they were in the form of single-digit round tubes, then multidigit tubes were developed, followed by flat multidigit panels.27 These devices are now used in a wide range of applications, including audio and video equipment, home electronics, office equipment, and in road vehicles.28... 

Morimoto, K. Pykosz, T. L. Trend in Vacuum Fluorescent Display. In Electronic Display and Information Systems and On-board Electronics, 820264, 1982, 69, p 103. 

Morimoto, K. Phosphors for Vacuum Fluorescent Displays and Field Emission Displays. In Phosphor Handbook. Shionoya, S. Yen, W. M. Eds. CRC Press New York, 1998, Chapter 8, pp 561-580. 

The best-known exception to Kasha s rule is the anomalous fluorescence displayed by azulene and its derivatives (nonaltemant hydrocarbons) and some aliphatic and aromatic thioketones. 

Vacuum Fluorescence Display non-select voltage (volts) operating voltage (volts) select voltage (volts) threshold voltage (volts) birefringence... 

Fig. 8.1S. A field-emission fluorescent display based on carbon nanotubes. The anode current is 200 pA at a voltage of 10 kV. Reproduced from ref [194], /ith permission.

Note that we have shown that phosphor excitation can occur via one of two paths, that of electron bombardment (cathode-rays) or by electric field generation of electrons. The former has been further diAdded into external (electron gun) and internal (electric field) generation of an electron beam. Since we have already described color TV CRT s, we will first address the other types of CRTs first and then internal electron generation in selected devices such as the vacuum fluorescent display. 

The vacuum fluorescent display is a type of CRT except that the electron beam is diffuse instead of being focused. Its construction consists of 1) an anode substrate, 2) an electrode assembly and 3) a front glass plate. One construction is shown in the following dlcigram, given as 7.2.14. on the next page. 

Note the rare earth based compositions are not represented. None of these phosphors, except the ZnO Zn composition have been entirely adequate. It is, perhaps, for this reason that this display has become nearly obsolete. With the advent of flat panel displays such as the computer controlled plasma display or the back-lighted liquid-crystal displays (which we will describe later), the vacuum fluorescent display has been largely superseded. Such computer controlled displays can show the item purchased, the amount of the sale and keep a running total for the buyer. 

At such voltages, the phosphors that can be used are limited and conductive phosphors seem to perform best. Although the type of phosphor mixtures given in 7.2.15. for vacuum fluorescent displays have been tried, none seem to be entirely satisfactory. The phosphors that have been tried or are being used in the FED display are ... 

A low level of vibrational excitation has also been observed in the CN(5 2 ) fragments produced through the photolysis of (CN)2 at 130.4 nm, 129.5 nm and 123.6 nm, with/v (the fraction of excess energy going into vibration of the CN(5) fragment) in the range 7-12% The (0,0) band of the CN(fi ->. Y) fluorescence displays none of the enhanced rotational intensities associated with the parallel formation of... 

The phenomenon of fluorescence displays a number of general characteristics. Exceptions are known, but these... 

Current flat-panel display technology primarily revolves aroimd inorganic LEDs, backlit liquid crystal displays (LCDs) and vacuum fluorescent displays. Although these technologies are trim and efficient compared with cathode ray tubes, they can be bulkier and much more power consuming than required for many applications. In many battery-operated devices, such as laptop computers, cellular telephones and other hand-held instruments, the illuminated display is the primary energy consumer. Furthermore, LED displays are expensive to fabricate because of the many individual diodes required to make up an alphabet, each with its own contacts and interconnections. These problems forced researchers to look for other materials. 

The display technologies traditionally employed in these applications are LCDs, vacuum fluorescence displays, and inorganic LEDs. Industrial soiu-ces estimate that hy 2004-2005, organic LEDs (including PLEDs as well as LEDs based on small organic molecules) may capture between US 350 to US 700 million of that market (3,35). In the long run, PLEDs also have the potential to replace current high resolution video-rate displays in, eg, desktop monitors, ultrathin television sets, and the like. 

References [181, 182] demonstrated fluorescent displays with the possibility of switching between two colors [181] and optical shutters, switched by short electrical pulses [182] (less than 1 ms). The color of fluorescent displays is readable in the dark at a low level of ambient light, the displays possess excellent viewing characteristics at oblique viewing and are insensitive to any irregularity in cell thickness. Fluorescent displays can compete with the usual guest-host devices in some indoor applications. 

According to estimations [14, 15, 16] to the end of this century the overall production cost of LCDs will come close to that of CRTs and even exceed the latter (and will considerably exceed that for their nearest competitors vacuum fluorescent displays (VFD), plasma displays (PD), and electroluminiscent displays (ED) (Fig. 8.4). 

The simplified scheme (Figure 68.25) for the photoinduced processes in salicylideneaniline molecules is based on spectroscopic work and quantum chemical calculations, together with well-known kinetic data for the excited state of molecule i. The primary photoinduced process, a fast ESIPT, takes place in the 1 state, leading to the formation of the planar NH structure (H), which is responsible for the anomalously large Stokes-shifted (ASS) fluorescence displayed only in rigid media. Two processes then compete with the ASS fluorescence adiabatic ring A twisting occurs only in liquids and leads to the TICT state [ (T), X = 90°] in the Z-structure of the NH form. This structure makes a considerable contribution... 

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